1. Technical Field
This invention relates generally to a sensor interface circuit and, more particularly, to a circuit and method for processing sensor signals output from an oxygen sensor such as that employed with an automobile while compensating for external input impedances and internal loads.
2. Discussion
A sensor interface circuit is generally employed to interface the analog outputs of a sensor with a processing device. This type of interface circuit is typically used to sufficiently amplify and condition the sensor signal as required for further processing. In automotive applications, an oxygen sensor is typically mounted in the exhaust system of an automotive engine, while an oxygen sensor interface circuit is usually mounted in an engine control module, powertrain control module or in combination with some other processing device. In accordance with well known automotive applications, the oxygen sensor signals are commonly used to maintain a stoichiometric air-to-fuel ratio for maintaining proper operation of the catalytic converter.
Since the oxygen sensor is typically located in the engine compartment and is separate from the interface circuit, the interface circuit generally must be capable of rejecting common mode signals. This is because input common mode voltage excursions may be as great as plus or minus two volts with respect to the ground employed by the interface circuit and processing circuitry coupled thereto. Also, in addition to providing a sufficiently amplified signal that may be properly processed, the interface circuit must also be capable of detecting various error conditions such as broken or open wires leading to the sensor.
Conventional oxygen sensor interface circuits have generally included an amplifier equipped with a fixed gain which receives an analog differential voltage across a pair of oxygen sensor output lines. The conventional circuit also includes a small voltage source for generating a known amplifier output which is indicative of either an open lead condition or an extremely cold sensor. One example of a commercially available sensor interface circuit is the sensor interface amplifier Model No. LM1964 which is manufactured by National Semiconductor. Another sensor interface circuit is disclosed in U.S. patent application Ser. No. 08/235,741, filed Apr. 29, 1994, titled "Oxygen Sensor Interface Circuit with Simplified Amplifier Requirements," and assigned to the assignee of the present invention. The above prior art examples are precision differential amplifiers designed for use in the automotive environment in which a sensor is typically grounded at the engine block while the sensor interface amplifier is grounded at chassis potential. The above examples further employ an external capacitor to filter out high frequency transients and typically exhibits an impedance of about 1 M.OMEGA..
For most practical applications, sensor interface circuits generally require input filtering to filter out unwanted noise, especially for oxygen sensors which are commonly located in close proximity to the engine of a vehicle. In the past, the input filtering has been accomplished by connecting a resistor-capacitor (RC) circuit to each input terminal. A typical RC filter circuit has a capacitor coupled to ground and a resistor connected in series between the sensor output line and the amplifier. While such an input filtering configuration operates to minimize high frequency noise, the RC filter circuit also introduces an AC impedance to ground on both the positive and negative inputs which can cause problems, especially when either the positive or negative leads are opened. In addition, gain attenuation problems are further introduced into the circuit. The gain attenuation problems are often caused by the RC filter circuit resistors reducing the differential voltage applied to the amplifier. This generally results in an error and reduces gain. Errors of this kind are undesirable, especially for current and future engine and powertrain control module (ECM and PCM) applications which generally require accurate sensing information.
Prior attempts have been made to correct or otherwise compensate for the above cited problems. To account for an open circuit, a current source has been added along with a comparator for detecting if the input voltage deviates from a common mode range. However, in accordance with conventional approaches, this has generally resulted in the presence of an offset at the output. In practice, a constant offset has been established at the output in excess of twenty millivolts. This amount of offset is excessive for most modern and future engine control modules and powertrain control modules.
Additionally, conventional sensor interface circuits such as that mentioned in the above example generally employ bipolar transistor and resistor circuitry. However, such conventional interface circuits tend to have a number of problems associated therewith. First, matching characteristics of commonly employed resistors are generally not accurate enough to easily meet the current and future needs of automotive oxygen sensor applications. These inaccuracies typically require that both the gain and input referred voltage be adjusted on most every part that is manufactured. This can lead to an expensive and time consuming process. Second, the input impedance of the interface circuit is usually a function of an internal resistor which generally has variations associated therewith due to process and temperature conditions. Variations in the input impedance can adversely affect the transfer response of a given system. Also, many conventional approaches generally employ operational amplifiers which are connected to a common mode ground. This has the effect of limiting the amplifier operation and complicating the amplifier requirements.
It is therefore desirable to provide for an interface circuit that interfaces with sensor output signals to process the signals and realize a desired transfer curve while compensating for external impedances and internal loads.
It is a further object of the present invention to provide for an oxygen sensor interface circuit which employs accurate matching characteristics which generally do not vary or otherwise adversely affect the transfer response of a given system.
It is another object of the present invention to provide for an oxygen sensor interface circuit which employs a switched capacitor circuit topology to achieve a desired transfer response and allows for external impedance matching.
It is yet another object of the present invention to provide an oxygen sensor interface circuit and method that employs input filtering with RC circuitry and provides compensation for added series resistance to minimize gain attenuation problems and reduce problems associated with an AC impedance to ground.